1. Field of the Invention
This invention relates to a friction material for a friction clutch device that has a single friction plate or a plurality of friction plates for use in an automatic transmission of an automobile, a transmission of a motorcycle or the like, and to its manufacturing method.
2. Description of the Related Art
A wet friction clutch such as a multiplate clutch has one or a plurality of wet friction plates that are used in a lubricant. There are wet friction plates that are made of a sintered alloy, a carbon or a cork. Still, a paper wet friction plate is called as “a paper friction material” and is commonly used.
In manufacturing the paper wet friction material, a paper body is made of a substrate fiber such as a pulp or an aramid fiber and a filler such as a friction adjuster or a base filler. Next, the paper body is impregnated with a resin binder made of a thermosetting resin. Then, the paper body is heated to cure or harden the resin binder, thereby resulting in the paper wet friction material. Such paper wet friction material is advantageous because it is lightweight and cheap. Moreover, the paper wet friction material is made of a porous material and relatively rich in elasticity with resultant high oil absorbency. Furthermore, the paper wet friction material is excellent in heat resistance, abrasion resistance and the like.
A resin existing near a friction surface of the friction material is one of factors that determine the heat resistance (particularly a heat spot resistance) of the friction material. If an amount of the resin near the friction surface is large, the friction material becomes hard and the heat resistance is lowered. Thus, it is thought that it is desirable for the resin amount to be small.
However, if the total resin amount in the friction material is reduced so as to decrease the resin amount near the friction surface, it is impossible to maintain strength required for the friction material. Therefore, a minimum amount of resin is mixed into the friction material so as to balance the heat resistance and the strength.
Still, it is indispensable to remove the resin at the friction surface in order to achieve an ultrahigh heat resistance required for the friction material in these years. As a means to solve such problem, a post-treatment process such as a surface polishing or a high heat resin degradation (heat shearing) is added to improve the heat resistance.
However, if such post-treatment process is added, the costs increase. Moreover, there take place adverse effects in case of the post-treatment process. That is, if the surface polishing is adopted, the friction face of the friction material becomes rough or fuzzy so as to increase a drag torque. On other hand, if the heat shearing is adopted, even an inside of the friction material reaches a high temperature so as to cause deterioration of strength or the like. Thus, it is very hard at present to attain goals recently required for the friction material such as the low costs and the ultrahigh heat resistance.